1. Field of the Invention
This invention relates generally to an antenna feed horn and, more particularly, to a compact, low weight antenna feed horn for a satellite communications antenna feed array or phased array, that includes multiple transition steps to provide multimode signal propagation, a relatively wide bandwidth having a low axial ratio, substantially equal E-plane and H-plane beamwidths, low cross-polarization and suppressed sidelobes.
2. Discussion of the Related Art
Various communications networks, such as Ka-band satellite communications networks, employ satellites orbiting the Earth in a geosynchronous orbit. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and then is switched and re-transmitted by the satellite to the Earth as a downlink communications signal to cover a desirable reception area. The uplink and downlink signals are transmitted at a particular frequency bandwidth and are coded. Both commercial and military Ka-band communications satellite networks require a high effective radiating isotropic power (ERIP) in the downlink signal, and an acceptable gain versus temperature ratio (G/T) in the uplink signal for the communications link. The ERIP and G/T require a high gain antenna system, providing a smaller beam size, thus reducing the beam coverage and requiring a multi-beam antenna system. The satellite is therefore equipped with an antenna system that includes a plurality of antenna feed horns arranged in predetermined configuration that receive the uplink signals and transmit the downlink signals to the Earth over a predetermined field-of-view.
The antenna system must provide a beam scan capability up to fifteen beamwidths away from the antenna boresight with a low scan loss and minimal beam distortion in order to compensate for the longer path length losses at the edges of the field-of-view. Multi-beam antenna systems that produce a system of contiguous beams by a reflector system with the plurality of feed horns require highly circular beam symmetry, steep main beam roll-off, suppressed sidelobes and low cross-polarization to achieve low interference between adjacent beams. For cellular satellite communication, a circularly polarized system is necessary because they do not need polarization tracking.
To accomplish the above-stated parameters, the antenna feed horns must be capable of producing beam radiation patterns that have substantially equal E-plane and H-plane beamwidths over the operating frequency band of the signal. The level of the cross-polarization and the difference between the E-plane beamwidth and the H-plane beamwidth in the communication signal determines the axial ratio of the signal. If the cross-polarization is substantially low and the E-plane and H-plane beamwidths are substantially the same, the axial ratio is about one and the signals are effectively circularly polarized. However, if the E-plane and H-plane beamwidths are significantly different, the signal is elliptically polarized and the signal strength is reduced, causing increased insertion loss and data rate loss of the downlink signal.
The usable bandwidth in the downlink signal or the uplink signal that is able to transmit information is defined by the content of the propagation modes of the signal, as determined by the phase orientation of the modes. These propagation modes include the transverse electric (TE) modes where the electric field lines are in the transverse plane of wave propagation, and the transverse magnetic (TM) modes where the magnetic field lines are in the transverse plane of wave propagation. The orientation of the electric and magnetic fields in the various TE and TM modes defines the mode content of the signal.
Typical conical horns provide only the TE.sub.11 mode, where the E-plane beamwidth was substantially less than the H-plane beamwidth. Therefore, when used to transmit or receive a circularly polarized signal, the signals were not circularly polarized, but were elliptically polarized. In order to reduce the axial ratio and provide a more circularly polarized beam, Potter horns and corrugated horns were developed in the art that generated substantially equal E-plane and H-plane patterns with suppressed sidelobes. The Potter horn is disclosed in Potter, P. D., "A New Horn Antenna With Suppressed Sidelobes and Equal Beamwidths," Microwave J., Vol. Xl, June 1963, pp. 71-78. The Potter horn is a conical shaped feed horn that includes a single step transition that provides for the propagation of the TM.sub.11 mode for equal E-plane and H-plane beamwidths and suppressed sidelobes. The corrugated horn is a conical shaped feed horn that includes a corrugated structure within the horn from the waveguide to the aperture that also provides equal E and H plane beamwidth and suppresses the sidelobes.
Although the configuration of the Potter Horn is generally successful for providing a desirable mode content with low cross-polarization and suppressed sidelobe levels, the Potter Horn generates signals that are limited by their useful bandwidth, on the order of 3%. The corrugated horn is able to provide wider bandwidth, however, it will be heavy and more costly to fabricate due the corrugated structure of the horn.
What is needed is a compact, light weight antenna feed horn that provides substantially equal E plane and H-plane beamwidths, low cross-polarization and suppressed sidelobes, but has a higher useful bandwidth than those feed horns known in the art. It is therefore an object of the present invention to provide such an antenna feed horn.